Broadcast signal transmissions are often subject to multi-path distortions, particularly where receiver systems are moving (e.g., automobile, cellular phone, etc.). Multiple receive antennas and diversity combining techniques have been used to reduce degradation in receiver performance due to these multi-path distortions. In contrast to receiver systems that use diversity combining techniques to address multi-path distortions, other receiver systems are designed to receive multi-signal diversity transmissions where two or more transmitters are used to generate multi-signal diversity transmissions.
With respect to broadcasts that utilize OFDM (orthogonal frequency division multiplex) transmissions, such as HD-Radio (High Definition Radio) broadcasts in the United States, DAB/DMB (Digital Audio Broadcast/Digital Multimedia Broadcast) and DVB (Digital Video Broadcasting) broadcasts in Europe, ISDB-T (Integrated Services Digital Broadcasting—Terrestrial) broadcasts in Japan, and DTMB (Digital Terrestrial Multimedia Broadcast) broadcasts in China, diversity techniques are also useful in improving reception of the broadcast signals. Diversity combining for OFDM transmissions typically utilize diversity combining in the frequency-domain for the OFDM signals. This frequency-domain diversity combining requires channel estimation, frequency adjustments, and timing synchronization in order to combine the received OFDM signals for diversity.
FIG. 1 (Prior Art) is a more detailed block diagram of an embodiment 100 for a digital broadcast receiver system that utilizes frequency-domain OFDM diversity combining circuitry 146. First receiver circuitry 142 includes radio frequency front-end (RFFE) circuitry 152 that down-converts a radio frequency (RF) signal received from the antenna 138 and provides a down-converted quadrature (I/Q) signal to the analog-to-digital converter (ADC) 154. The ADC 154 provides a digitized signal to channelize circuitry 156, which in turn provides a channelized I/Q signal to the digital mixer 158 within the frequency-domain diversity combining circuitry 146. Similarly, second receiver circuitry 144 includes RFFE circuitry 162 that down-converts an RF signal received from the antenna 140 and provides a down-converted quadrature (I/Q) signal to the analog-to-digital converter (ADC) 164. The ADC 164 provides a digitized signal to channelize circuitry 166, which in turn provides a channelized I/Q signal to digital mixer 168 within the frequency-domain diversity combiner 146.
The frequency-domain OFDM diversity combining circuitry 146 includes mixers 158 and 168, fast Fourier transform (FFT) circuitry 160 and 170, synchronization (SYNC) block 172, channel estimators 174 and 184, ratio blocks 176 and 186, mixers 178 and 188, and combiner 180. The frequency-domain OFDM diversity combining circuitry 146 provides a combined I/Q receive signal 148 as an output. For the embodiment depicted, the combined I/Q receive signal 148 is then provided to de-mapper circuitry 190, deinterleave circuitry 192, and FEC (forward error correction) decode circuitry 194 before being provided as decoded output signals 196. These decoded output signals 196 can then be further processed by additional circuitry.
In operation, synchronization (SYNC) block 172 receives output signals from the mixers 158/168, provides a first frequency adjustment signal (f1SYNC) 157 back to the mixer 158, and a second frequency adjustment signal (f2SYNC) back to mixer 168. The SYNC block 172 also generates a first timing synchronization signal (T1SYNC) 161 that is applied to the FFT circuitry 160. FFT circuitry 160 also receives the output signal from mixer 158. The output from FFT circuitry 160 is provided to channel estimator 174, which in turn provides a signal to complex conjugation block (R1) 176 to generate a weighted mixing signal that is mixed with the output signal from FFT circuitry 160 by mixer 178. Similarly, the SYNC block 172 generates a second timing synchronization signal (T2SYNC) 171 that is applied to the FFT circuitry 170. The FFT circuitry 170 also receives the output signal from mixer 168. The output from FFT circuitry 170 is provided to channel estimator 184, which in turn provides a signal to second complex conjugation block (R2) 186 to generate a weighted mixing signal that is mixed with the output signal from FFT circuitry 170 by mixer 188. It is noted that conjugating the channel response using blocks (R1/R2) 176/186 phase aligns the two antenna signals so that they can be added coherently and weights each of these signals so that each contributes to the sum in proportion to its signal-to-noise ratio (SNR). The output if mixers 178/188 are provided to combiner 180, and combiner 180 generates a combined frequency-domain I/Q receive signal 148. It is noted that the frequency adjustment and timing synchronization provided by the SYNC block 172 is needed so that frequency-domain diversity weighting adjustments can be made using mixers 178 and 188 and the weighted mixing signals from the ratio blocks (R1, R2) 176 and 186.
One disadvantage with frequency-domain diversity combining, such as shown with respect to FIG. 1 (Prior Art), is the complexity and size required for the frequency-domain OFDM diversity combining circuitry 146, which requires channel estimation, related frequency adjustments, and timing synchronization in order to apply frequency-domain diversity combining to the received OFDM signals.